1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a method for fabricating an array substrate of an LCD device, to decrease the number of masks used.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been widely used due to the advantageous characteristics of high contrast ratio, improved gray and image display and low power consumption. To operate the LCD device, it is necessary to form various patterns of driving devices or lines on a substrate of the LCD device, wherein photolithography is generally used to form patterns on the substrate. To form the patterns on the substrate by photolithography, a photoresist coats the substrate to sense ultraviolet rays, the photoresist is exposed and developed to form a pattern using a mask on the photoresist, the material layers are etched by using the patterned photoresist as a mask, and then the photoresist is stripped.
In an array substrate for the LCD device according to the related art, it requires 5 to 7 masks to form a gate line layer, a gate insulating layer, a semiconductor layer, a data line layer, a passivation layer and a pixel electrode, whereby the probability of process errors increase and process yield decrease. In order to overcome these problems, low-mask technology has been actively studied, which improves productivity and obtains the process margin by fabricating the array substrate using the minimum number of masking and photolithography steps.
Hereinafter, a method for fabricating an array substrate of an LCD device according to the related art will be described with reference to the accompanying drawings.
FIG. 1 is a flow chart illustrating a method for fabricating an array substrate of an LCD device according to the related art. FIG. 2A to FIG. 2C′ are plane and cross-sectional views illustrating the fabrication process of an array substrate according to the related art.
To form the array substrate of the LCD device according to the related art, as shown in FIG. 1, a metal layer is deposited on a substrate, to form a gate line layer (S11 and S12). Then, a gate insulating layer is formed on an entire surface of the substrate including the gate line layer (S13), and an active layer is formed on the gate insulating layer overlapping with a predetermined portion of the gate line layer (S14). Next, a data line layer is formed to make a predetermined pattern with the gate line layer (S15), and a passivation layer having a contact hole is deposited on the data line layer (S16). After that, a pixel electrode is connected with a predetermined portion of the data line layer through the contact hole (S17). As a result, the array substrate for the LCD device is completed. The array substrate requires 5 masks in the steps of S12, S14, S15, S16 and S17.
A method for fabricating the array substrate for the LCD device according to the related art will be described in detail below.
First, as shown in FIGS. 2A & 2A′ a low-resistance metal layer, e.g., copper Cu, aluminum Al, aluminum neodymium AlNd, molybdenum Mo or chrome Cr, is deposited on a glass substrate 11, and photolithography using a first mask is carried out to form a plurality of gate line layers, including a gate line 12 and a gate electrode 12a. 
Photolithography includes forming the low-resistance metal layer on the transparent glass substrate having great heat-resistance characteristics and depositing a photoresist thereon. Then, the patterned first mask is positioned over the photoresist, and light is selectively irradiated thereto, whereby the same pattern as that of the first mask is formed on the photoresist. Next, the photoresist irradiated with the light is removed by an etchant, thereby leaving a photoresist pattern.
For reference, the etching process may be classified as a dry-etching method and a wet-etching method, wherein the dry-etching method removes a lower layer exposed below the photoresist with plasma gases or radicals, and the wet-etching method uses a chemical solution. Also, the dry-etching method is generally used to etch an insulating layer, which requires an accurate pattern. The wet-etching method is generally used to etch a metal material or a transparent electrode, and the wet-etching method uses low-priced fabrication equipment and has great productivity.
Next, an inorganic layer of silicon nitride SiNx or silicon oxide SiOx is deposited on the entire surface of the substrate including the gate line 12 at a high temperature, thereby forming a gate insulating layer 13. Subsequently, an island-shaped semiconductor layer 14 is formed by photolithography using a second mask on the gate insulating layer 13 overlapping the gate electrode 12a. At this time, the semiconductor layer 14 is formed by depositing an amorphous silicon (a-Si:H) at a high temperature. The gate insulating layer 13 and the semiconductor layer 14 are generally deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition), which is carried out at a temperature of approx. 250° C. or more.
Referring to FIGS. 2B & 2B′, a low-resistance metal layer, for example, copper Cu, aluminum Al, aluminum neodymium AlNd, molybdenum Mo or chrome Cr is deposited on the entire surface of the substrate including the semiconductor layer 14 and patterned by photolithography using a third mask, thereby forming a data line layer. The data line layer includes a data line 15 substantially perpendicular to the gate line 12, and source drain electrodes 15a and 15b overlapping both sides of the semiconductor layer 14. The deposited gate electrode 12a, the gate insulating layer 13, the semiconductor layer 14 and the source/drain electrodes 15a and 15b form a thin film transistor controlling a data voltage applied to the unit pixel region.
Next, as shown in FIGS. 2C & 2C′ an organic insulating layer of BCB or an inorganic insulating layer of SiNx is deposited on the entire surface of the substrate including the data line 15, to form a passivation layer 16. Then, some of the passivation layer 16 is selectively removed by photolithography using a fourth mask, thereby forming a contact hole exposing the drain electrode 15b. Also, a transparent conductive layer of ITO (Indium-Tin-Oxide) or IZO (Indium-Zinc-Oxide) is formed on the entire surface of the substrate including the passivation layer 16, and patterned by photolithography using a fifth mask, whereby a pixel electrode 17 is electrically connected with the drain electrode 15b, thereby completing the array substrate of the LCD device. Thereafter, although not shown, the array substrate forming the thin film transistor TFT is bonded to an opposing substrate by a sealant with spacers between the two substrates. Then, liquid crystal is injected between the two substrates, to form a liquid crystal layer, and then an inlet for injection of liquid crystal is sealed, thereby completing the LCD device.
However, the array substrate for the LCD device according to the related art and the method for fabricating the same have following disadvantages.
The method for fabricating the array substrate of the LCD device according to the related art requires 5 masks when forming the gate line layer, semiconductor layer, the data line layer, the contact hole of the passivation layer and the pixel electrode, thereby lowering fabrication efficiency due to the complicated fabrication process and the increase of fabrication time and cost.